This invention relates generally to a method and apparatus for producing hydrocarbons from formations comprising hydrocarbon hydrates and relates more particularly to a substantially self-powered method and apparatus for such production.
Methane and other hydrocarbons are known to react with liquid water or ice to form solid compounds which contain both water and individual or mixed hydrocarbons. For example, if the methane pressure is 400 pounds per square inch (psi) and the water temperature is 32.degree. F., then methane hydrate can form. Likewise, at 2000 psi and 60.degree. F., the solid hydrate will form. For hydrocarbons to react with brine (defined herein as any solution based on a water solvent), as opposed to pure water, the methane pressures must be slightly higher at a given temperature; but in other ways the behavior is very similar. The hydrate compositions vary a little depending upon the conditions of formation, CH.sub.4.5.75H.sub.2 O and C.sub.3 H.sub.8.17H.sub.2 O being two compositions which can form. The hydrates are slightly less dense than ice.
Natural conditions suitable for the formation of solid hydrocarbon hydrates exist in a shell covering much of the earth which lies between about 1000 and a few thousand feet below the earth surface. However, at the earth surface the hydrocarbon pressure is too low for the hydrates to exist; and deep in the earth the geothermal gradient leads to temperatures too high for the hydrates to exist. On the ocean floor, the forming of a hydrate will yield an ice-like solid which will float up and be destroyed unless the material is anchored by a more dense material, for example mud or a porous formation (e.g., sandstone). However, near-freezing water or brine does exist widely below the earth surface beneath formations which will anchor the solid hydrates; and methane amd other gaseous hydrocarbons are constantly generated deeper in the earth as buried organic material is thermally decomposed as it sinks slowly into geothermal zones. Excellent conditions for formation of methane hydrate and other hydrates exist on muddy ocean floors where cold, dense brine settles at pressures over 400 psi and buried alluvial or deltaic material is generating methane. Sonic and other measurements suggest that very extensive hydrocarbon hydrate resources exist in the ocean depths off the coast of the eastern United States and elsewhere, often in the form of frozen muds which release their methane if they are heated.
Therefore, a very important problem today is how to recover natural gas economically from such hydrate formations.
Russian scientists have considered such hydrates, especially underground in the Siberian permafrost regions, as attractive sources of natural gas, (as disclosed in Yu. F. Makogon, "Hydrates of Natural Gas," Geoexplorers Associates, Inc., Denver, Col., 1978). That reference suggested (on page 155 ) decomposing such underground hydrates by heating the hydrate deposit from below the reservoir using geothermal waters. However, no details are given. And it is believed that they and others have not achieved either the method or apparatus of this invention for recovering hydrocarbons in a substantially self-powered manner.
Russian workers have reported that they have obtained methane from the underground hydrates by drilling into the hydrates and then injecting methanol or salts to melt the hydrates. See, for example, Yu. F. Makogan (cited above) at page 127. See also W. J. Cieslewicz, "Some Technical Problems and Developments in Soviet Petroleum and Gas Production", The Mines Magazine, Nov. 1971, pp. 12-16, at 15, where three methods of converting solid hydrate into the gaseous state directly in the formation were listed. The three methods included (1) pumping of catalysts (e.g., methanol) into the formation, (2) artificially reducing formation pressure, and (3) increasing formation temperature by pumping water, steam, or hot gases into the deposit (the method showing the best economic prospects in many areas of Siberia which have abundant supplies of thermal waters). However, no details of the techniques were provided. And methanol or salt additions cool down, rather than heat, the deposits; and, in consequence, the methane recovery is delayed or limited. Furthermore, introducing any liquid into hydrates by conventional (as opposed to self-powered) pumping would be expensive, often prohibitively so.
Additionally, many others have addressed producing methane and other hydrocarbon gases which are dissolved in brine or water, as opposed to occurring as solids, particularly geopressured-geothermal (GPGT) brine which can be delivered to the surface by artesian forces, thereby permitting above-ground processing. However, the hot geothermal brines prevent the formation of the solid hydrates of hydrocarbons which are of interest in the present patent application. And the methods of recovering dissolved methane (particularly the economically promising methods involving pressure reduction) have little relationship to the present invention for recovering methane from solid hydrates.
Although it is well known in the art to melt solid sulfur in the Frasch process (as described, for example, by Linus Pauling in College Chemistry, W. H. Freeman and Co., 2nd Edition, 1957, on pages 299-300, wherein water superheated to above the sulfur melting point (about 119.degree. C.) is pumped under pressure into the sulfur deposits), the Frasch process is not self-powered and the product recovered is a solid, not a gas. Additionally, pumping from the surface by conventional methods and devices is expensive.
Therefore, a need still exists for a substantially self-powered device and method for economically recovering hydrocarbons (including methane) from deposits of natural gas-containing hydrates which are solid formations or which contain substantially solid hydrates (e.g., in the form of a slush).